The human spine is composed of cervical, thoracic, lumbar and sacrum vertebrae. The neck region of the spine is known as the cervical spine, which consists of seven vertebrae, which are abbreviated C1 through C7 (top to bottom). Beneath the last cervical vertebra are twelve vertebrae of the thoracic spine. The thoracic vertebrae are abbreviated T1 through T12 (top to bottom). Beneath the last thoracic vertebra are five lumbar vertebrae, abbreviated L1 through L5. The size and shape of each lumbar vertebra is designed to carry most of the body's weight. Each structural element of a lumbar vertebra is bigger, wider and broader than similar components in the cervical and thoracic regions. Beneath the lumbar spine, five sacral vertebrae, abbreviated S1 through S5, grow together to form the triangular bone called the sacrum. The lowest four vertebrae beneath the sacrum form the tailbone or coccyx.
A typical vertebra consists of two essential parts, i.e., an anterior (front) segment, which is the vertebral body, and a projectionerior part which encloses the vertebral foramen. When the vertebrae are articulated with each other, the bodies form a strong pillar for the support of the head and trunk.
The cervical, thoracic and lumbar vertebrae and sacrum are separated by intervertebral discs. For example, the intervertebral disc located at the interface between the largest lumbar vertebrae (L5) and the first sacral vertebrae (S1) is commonly referred to as the L5/S1 disc. An intervertebral disc is composed of a tough, fibrous ring, commonly referred to as the annulus fibrosus, which is wrapped around a jelly-like center, commonly referred to as the nucleus pulposus.
The intervertebral discs are exposed to a variety of forces and stress depending upon the position and movement of the body, and position of the disc within the body. Because the spine supports the weight of the body, axial compressive forces are constantly applied to the discs, in either a standing or sitting pose of the body. Furthermore, the intervertebral discs are exposed to additional forces and stress resulting from contortion of the body, for example, flexion (i.e., bending forward), extension (i.e., bending backward), lateral bending (i.e., bending to either side), and torsional movement (i.e., upper body twist) of the body. Furthermore, discs that rest on an angled plane are exposed to shear stress. Generally, the steeper the angle of the disc, the greater the shear stress imparted on that disc. For example, the L5/S1 disc resides on the steepest plane of all the discs of the spine. It follows that the L5/S1 disc is exposed to the greatest compressive force and shear stress of all the spinal discs because the L5/S1 disc is the lowest disc in the spine. For the foregoing reasons the L5/S1 disc is particularly prone to bulging, herniation, deformity, or deterioration under the aforementioned compressive force, shear stress, or trauma.
Pain caused by a herniated, bulging, deformed, or disintegrated lumbar disc at either the L4/L5 or L5/S1 is commonly known as sciatica. The sciatic nerve exits the spinal column between the lowest lumbar vertebral body (L5) and first vertebrae of the sacrum (S1). Compression, inflammation or irritation of the sciatic nerve due to contact with a herniated disc or misaligned vertebrae is a common cause of pain. Discectomy surgery to remove the L5/S1 disc and replace it with a fusion ALIF spacer is often employed to relieve the pain of sciatica.
The degenerated L5/S1 disc (or any other lumbar disc) may be replaced with an anterior lumbar interbody fusion ALIF spacer, hereinafter ALIF spacer. The ALIF procedure is similar to a posterior lumbar interbody fusion (PLIF), except that in an ALIF procedure the disc space is fused by approaching the spine through the abdomen instead of through the lower back.
Common ALIF spacers include a hollow center that is pre-filled with bone graft, such as an autograft, allograft, synthetic bone substitute, or bone morphogenic protein (BMP). Such a graft is intended to accelerate the biological fusion of adjacent vertebrae. Specifically, nutrient blood vessels which form between the vertebrae and within the ALIF spacer stimulate bone growth. In the presence of the nutrient blood vessels, the vertebrae fuse with the graft and ultimately fuse together. Furthermore, it has been found that the nutrient blood vessels form more rapidly when the adjacent vertebrae are appropriately loaded.
In an unspecified percentage of ALIF procedures, the vertebrae fail to fuse together due to either lack of nutrient blood vessel generation or lack of axial compressive load between the vertebrae (stress shielding). It is generally held that excessive extension, flexion, torsion, lateral movement, and shear stresses applied to the ALIF spacer interrupt formation of the nutrient blood vessels, thereby inhibiting fusion of the vertebrae.
In the interest of improving discectomy procedures, particularly ALIF procedures, vertebral stabilization devices have been devised to maintain the graft in a state of compression as well as restrict flexion, extension, lateral, torsional, and shear motion of the adjacent vertebrae (a diagram of the motions are illustrated in FIG. 1B).
In general, three types of stabilization devices currently exist. First, stabilization devices that support all of the vertebra rendering no force on the intervertebral graft are called “stress shielding” devices. Second, stabilization devices that support or share a portion of the spinal load in parallel with the graft are called load sharing devices. Finally, stabilization devices that allow axial subsidence of the implant and support most of the load on the individual bone grafts are referred to as dynamized stabilization devices. This invention focuses on dynamized stabilization devices. For example, a dynamized stabilization device is disclosed in U.S. Pat. No. 6,645,207 to Dixon et al., which is incorporated herein by reference in its entirety. However, a need exists for a dynamized stabilization device that conforms to the unique anatomical constraints of the lower lumbar region of the spine.